In situ manipulation of catalyst performance via the photocontrolled aggregation/dissociation state of the catalyst

Article abstract of DOI:10.1039/c3cc00008g

A diamide armed with a photochromic azobenzene unit and a nucleophilic pyridyl group varied its aggregation/dissociation state depending on the trans/cis geometry of the azobenzene. The catalytic activity of the pyridyl group was closely linked to the aggregation/dissociation state and hence photoirradiation could manipulate the catalytic performance. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3cc00008g

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In situ manipulation of catalyst performance via the
photocontrolled aggregation/dissociation state of the
catalyst†
Cite this: Chem. Commun., 2013,
49, 4628
Received 14th March 2013,
Accepted 2nd April 2013
Akihiro Nojiri,ab Naoya Kumagai*a and Masakatsu Shibasaki*ac
DOI: 10.1039/c3cc00008g
www.rsc.org/chemcomm
A diamide armed with a photochromic azobenzene unit and a
nucleophilic pyridyl group varied its aggregation/dissociation
state depending on the trans/cis geometry of the azobenzene.
The catalytic activity of the pyridyl group was closely linked to the
aggregation/dissociation state and hence photoirradiation could
manipulate the catalytic performance.
Catalysis plays a pivotal role in the efficient production of bulk
and fine chemicals.1 In practice, catalysts are specifically designed
and fine-tuned for optimal execution in certain target reactions.
Besides this reaction-oriented pursuit of catalysts, multistate
catalytic systems, which produce differential catalytic functions
and/or performance depending on the state of the catalyst, have
received increased attention.2,3 Various external triggers, such as
photoirradiation and chemical additives, have been exploited to
induce structural/conformational changes in the catalyst and
hence different catalytic functions can be produced.4
Recently, we showed that bis(2-hydroxyphenyl)amide framework5
1a exhibits heterochiral aggregation through a hydrogen bond in
halogenated solvents, where (R)- and (S)-1a were mixed together to
form insoluble precipitates while homochiral (R)- or (S)-1a remained
in solution (Fig. 1a).6 The two amide and phenol functionalities are
Fig. 1 Background of aggregation/dissociation property of bis(2-hydroxyphenyl)-
amide framework.
Design of the multistate catalyst (S)-1c is shown in Fig. 2a. The
responsible for the aggregation, and compound 1b harnessed with a diamide framework and central a-amino acid residue part are
photochromic azobenzene unit allowed for reversible aggregation/ engaged for aggregation/dissociation and photochromic control,
dissociationbyUV/Visirradiation(Fig. 1b).7 Herein, we envisaged that respectively. Therefore, a catalytic function was introduced into the
incorporation of a certain catalytic function into the photochromic aromatic group located at both sides. We selected a nucleophilic
diamide framework could produce a photoresponsive multistate pyridyl group as a catalytic part and replaced the anilide benzene ring
catalyst (Fig. 1c). Aggregation/dissociation capability can be applied on the right side with a 4-pyridyl group for highest nucleo-
to the in situ manipulation of catalyst concentration in the solution philicity.8 In the initial reaction screening, the presence of two phenol
phase, which should be directly linked to catalytic activity, and hence groups frequently impeded the target reactions. In contrast, two
catalytic performance can be controlled by UV/Vis irradiation.
phenol groups were indispensable for the heterochiral aggregation of
1a in halogenated solvent; the newly designed diamide trans-(S)-1c
installed with a 4-pyridyl group lacking two phenol groups exhibited
inherently lower solubility compared with 1b, and even homochiral
trans-(S)-1c formed the aggregates in acetonitrile. The aggregates
were dissociated leading to a homogeneous solution when UV
irradiation (365 nm) was used to photoisomerize the azobenzene
unit from trans to cis. The contrast of the solubility difference
between trans-1c and cis-1c was further enhanced in a mixed solvent
a Institute of Microbial Chemistry (BIKAKEN), 3-14-23 Kamiosaki, Shinagawa-ku,
Tokyo 141-0021, Japan. E-mail: nkumagai@bikaken.or.jp,
mshibasa@bikaken.or.jp; Fax: +81-3-3441-7589
b Graduate School of Pharmaceutical Sciences, The University of Tokyo,
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
c JST, ACT-C, 3-14-23 Kamiosaki, Shinagawa-ku, Tokyo 141-0021, Japan
† Electronic supplementary information (ESI) available: Experimental procedures
and characterization of new compounds. See DOI: 10.1039/c3cc00008g
c
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Fig. 3 Profile of the reaction of 2 with 3 promoted by trans-1c (blue square) and
cis-1c9 (red circle). 2: 0.1 mmol, 3: 0.12 mmol, solvent 1.5 mL.
Fig. 2 Design and property of multistate catalyst 1c. UV (365 nm) irradiation
was used during the period 0–150 min and 195–325 min. Visible light (>422 nm)
irradiation was used during the period 165–180 min and 340–355 min. Green
bars and numbers represent total amount of 1c in solution phase.
led us to attempt the in situ manipulation of the catalytic perfor-
mance of 1c by a real-time transition of the aggregation/dissociation
state of 1c (Fig. 4). The reaction was initiated with 15 mol% of cis-1c9
to promote the formation of 4 in a homogeneous mixture. At 30 min,
analysis of the reaction mixture showed that 4 was produced in 19%
system of n-hexane–propionitrile = 4/1 (Fig. 2b). Only 4% of 1c (trans/ yield (black square) and the content of trans-1c (blue bar) and cis-1c
cis = 100/0, trans-1c: blue square) was present in the solution phase. (red bar) in the solution phase was 5.6% and 94%, respectively. At
Upon UV (365 nm) irradiation, the fraction of cis-1c (red circle) this point, visible light irradiation (>422 nm) was used to induce
gradually increased and a homogeneous solution developed (at photoisomerization from cis-1c to trans-1c. The reaction mixture
165 min). Visible light (>422 nm) led to re-isomerization to trans-1c gradually became turbid because of aggregation of trans-1c, and the
and 89.5% of 1c aggregated to be excluded from the solution phase reaction of 2 with 3 was retarded.
(at 195 min). Aggregation/dissociation could be repeated.
At 90 min, almost all of the cis-1c was isomerized to trans-1c and
With diamide 1c harnessed with a nucleophilic catalytic unit and the irradiation was terminated. Catalyst 1c was extensively aggre-
reversible aggregation/dissociation capability, we explored the cata- gated, which shut off its catalytic activity and only 6.4% of 4 was
lytic performance of 1c. An initial trial was attempted in the reaction produced over a 120 min period (from 90 min to 210 min). Next,
of 1-naphthol (2) with Boc2O (3), which can be accelerated in the the reaction mixture was irradiated with UV light (365 nm) to
presence of a nucleophilic catalyst. To enhance the contrast of re-isomerize trans-1c to cis-1c, inducing the dissociation of the
aggregation/dissociation states of 1c, the reaction media were exam- catalyst to revive its catalytic performance. After 90 min of irradiation
ined, which indicated that n-hexane–propionitrile = 4/1 was best (at 300 min), the content of cis-1c increased to 33% and the reaction
suited in the present system; substrates 2, 3, and cis-1c9 were rate of 2 with 3 gradually increased achieving steady production of 4.
perfectly miscible while the solubility of trans-1c was limited to only UV or visible light failed to promote the reaction, confirming that the
ca. 1 mM. Initially, the catalytic performance of trans-1c and cis-1c9 reaction progress was manipulated solely by the aggregation/disso-
was evaluated individually in the reaction of 2 with 3 with 15 mol% ciation state of 1c. This catalytic system was applicable to the
of catalyst loading (Fig. 3). With the soluble cis-1c,9 the reaction rearrangement of 2-acyloxybenzofuran 5 to 3-acyl-2-benzofuranone
profile showed steady progress at 21 1C to give carbonate 4, and the 6 that can be promoted by a nucleophilic catalyst (Fig. 5).8c,11 In
reaction mixture remained homogeneous. In marked contrast, the CHCl3 at room temperature, where both trans-1c and cis-1c9
reaction using trans-1c under otherwise identical conditions barely were miscible, the rearrangement of 5 was promoted by
produced 4 in the same reaction period, because the extensive 25 mol% of either trans or cis-1c9 at nearly identical reaction
aggregation of trans-1c caused significant deterioration of the cata- rates.10 In the n-hexane–ethyl acetate = 1/1 mixed solvent
lytic performance. Trans-1c and cis-1c9 produced comparable cataly- system, trans-1c showed limited solubility (ca. 1 mM) because
tic performance in CHCl3,10 where both trans-1c and cis-1c9 were of extensive aggregation and only 14% of rearranged product 6
miscible, indicating that the differential catalytic performance was was obtained after 100 min of reaction. On the other hand, with
ascribed to the aggregation/dissociation of the catalyst. This result 25 mol% of cis-1c,9 the reaction mixture was homogeneous and
c
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This work was financially supported by KAKENHI
(24106745) from MEXT. A.N. thanks JSPS for a predoctoral
fellowship.
Notes and references
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rapid reaction progress was observed to afford 80% of 6 after
100 min.12,13
In conclusion, we developed a photoswitchable nucleophilic
catalyst based on aggregation/dissociation of a diamide frame-
work. The catalyst on–off state could be manipulated in the
course of the reaction by UV/Vis irradiation. Installation of
other catalytically active functional groups to produce distinct
photoswitchable catalysts is currently underway.
5 For general utility of 1a and its analogue in asymmetric catalysis:
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9 Trans/cis = 4/96 at the photostationary state.
10 See ESI† for details.
11 I. D. Hills and G. C. Fu, Angew. Chem., Int. Ed., 2003, 42, 3921.
12 6 was obtained in 7.8% ee and 14.5% ee by trans-(S)-1c and cis-(S)-1c,
respectively (the (S)-isomer was obtained as a major product).
13 A small amount of 3-phenyl-2-benzofuranone was associated as a
byproduct.
Fig. 5 Profile of the rearrangement of 5 to 6 promoted by trans-1c (blue square)
and cis-1c9 (red circle). 5: 0.06 mmol, solvent 1.5 mL.
c
4630 Chem. Commun., 2013, 49, 4628--4630
This journal is The Royal Society of Chemistry 2013